Known very high temperature calibration furnaces are used calibrating thermal instrumentation such as optical pyrometers, radiometers, thermal scanners and thermocouples. For these applications, a tubular heating element includes a hollow tube of graphite open at both ends and having an integral centrally disposed partition oriented at right angles to the axis of elongation of the tube. The tube thus consists of dual back to back symmetrical cavities which have opposite open ends and a common closed end formed by the partition. When the element is electrically energized, these cavities function as blackbodies which radiate equal amounts of radiation in opposite directions through the open ends of the cavities, the radiation essentially satisfying the well known Planck formula for blackbody radiation.
In order for these cavities to closely approach the ideal black body described by the Planck formula, it is necessary for all internal walls to be maintained at a uniform temperature. In addition, the surface emittance must be large and the length L of the cavity is at least equal to its diameter D. which results in an apparent emissivity approaching 1.00 at both mouths of the heated zone.
Known blackbodies having dual cavities made of graphite exhibit large thermal gradients and only contain internal walls at uniform temperature in a very small region, wherein the length L of the region is equal to the diameter D of the partition and the apparent emissivity of this area is equal to about 0.98. The apparent emissivity is the emittance measured at the opening of the heated zone.
In order to improve the apparent emissivity of such blackbodies, it is necessary to increase the length of the region which contains internal walls at uniform temperature.
The present invention is directed toward dual cylindrical blackbody structures made of graphite which exhibit improved performance by minimizing thermal gradients.